usertron2020 - many thanks for the detailed reply. Two points to respond to there myself.
Ref Halifax, the 'obsession' with him is that he was clearly the alternative candidate to Churchill in OTL in May 1940, and favoured by many at the top of the Conservatives (and, probably, the Palace). He'd had a successful period as Viceroy of India and was clearly capable of doing the job.
His being a peer wasn't an insurmountable objection though it was clearly a disadvantage. There were constitutional possibilities that could have enabled him to speak in the Commons, though not immediately (either a bill passed specifically to enable that, or else his peerage could have been put into abeyance and he then enters via a by-election). Either way, had he been the best candidate, he'd have got the job - you don't lose a world war for the sake of a constitutional nicety.
No one else has said this, so I will: Halifax's appointment, naming as the Prime Minister the Appeaser Supreme short of Horace Wilson and Chamberlain himself, sends the message to the outside world that Britain has decided on a "Peace At Any Price" policy once again. NO ONE interested in stopping the Axis (which includes the USSR at this point) would give Halifax the time of day. Roosevelt is unlikely to even return his calls.
Most importantly of all, it confirms to the world that the British cannot be trusted. NO L-L, not one dime, not one bullet, not one stick of butter. If war breaks out again (as it easily could if Benny the Moose decides to go haring off on his own), Britain is truly alone, fighting only with its own resources.
Unless....Halifax resigns the day the war starts up again. But Jesus, you'd need someone as Dark Horse as Randolph Churchill

to be put forward, even if only as a figurehead PM. If you want L-L to flow, anyway.
As for dark horses, beyond Eden, Duff Cooper was a former cabinet minister who resigned over Munich but there were precious few others. Nearly all the top end of the Tories were behind Chamberlain, so if you rule out Halifax on that basis then you also rule out Simon, Hoare and the like. One possible dark horse would be to look outside the Tory party altogether and go with Attlee as head of a national government (Lloyd George was the head of the smaller part of his coalition in 1916-18 so the precedent was there). Arguably Ernest Bevin would have made a better leader but he wasn't a possibility in 1940 - he might have been in 1943-5 though.
What was Atlee's record on Appeasement? And the Liberal Leader's?
Re Stalin and the collegiate types, I'm not sure I'd agree - 1953-6 was undoubtedly a collective leadership, which included Molotov. I'm not sure it would have been possible pre-1941 but had Stalin been arrested and executed immediately after the opening of Barbarossa then it would have made eminent sense to deal with the crisis in hand.
Even in 1953-56 the horrors of Stalin and his purges still lay burned on the Soviet psyche, even the Politburo's. That's why Krushchev's small denunciation of Stalin at the 20th Party Conference in 1956 was so vital.
Douglas-Home dropped his peerage when he became PM. "Unconstitutional" in the Westminster system isn't as ironclad as it is in the States.
I wouldn't equate 1963-1964 with 1940-41 for the UK.
I will offer a unconventional answer. Hitler would probably be the worst leader to be killed for the Allies.
Only if you mean in terms of pure military matters. The civilian populace of Occupied Europe would disagree with you.
I feel Charles de Gaulle and Chiang Kai-chek should be in the list too as leaders of the two other great powers that won the war and earned their countries a place in the UN security council.
Heck, without the French defense at Bir-Hakeim, the Germans would've reached El-Alamein like 5 days earlier and that battle would've been much more difficult for the Brits. Free France was the only of the Allies to have troops on the Eastern front. Small things that can change a war.
Switch Mao and the PRC for Chiang and the KMT and you have a deal. de Gaulle's real contributions were after WWII, not during it. Not that the FF didn't make their presence known.
There was also a theoretical essai by Charles de Gaulle which described possible means of action in a mechanized war, and was read by German generals but more or less ignored by the French.
A prophet is not without honor except in his own country. Why do you think they burned Joan of Arc?
Someone else would show up instead of de Gaulle to lead the French who were not enchanted with the notion of being a German pendant.
Chiang? Seriously? One could say he would be best to get rid of.
Agreed on both counts. But de Gaulle was special. I can think of half-a-dozen Chinese generals and warlords who could have done a better job than Chiang.
There's also the fact that there was going to have to be a national government and the Labour party had made it quite clear they wouldn't form one under Halifax.
Helps explain why they swept at the polls after VE-Day.